All lasers have thermal problems of some sort. In addition, gas lasers, particularly those using helium as one of the gases, suffer from gas leakage through the laser body walls. Some glass, such as fused silica, has characteristics desirable for use in lasers but the leakage problem effectively prevents its use because the loss of helium causes the laser to fail. Glass is also porous to gases other than helium, although to a lesser degree. Particularly troublesome in this regard is water vapor. Water vapor soaks into the structural interstices of the glass, which is a sponge-like on a microscopic level, and is released into the laser as the material heats. This mechanism is responsible for another common laser failure mechanism, oxygen contamination, as the water is broken into its constituents by the laser plasma. All gas lasers use some form of extra oxygen getter, generally a flashed barium compound to try to control this phenomenon, but the results are marginal.
Many gas lasers today use pyrex bodies, metal-to-glass end seals for mirror mounting, and the flashed barium getters for oxygen control. All of these measures slow but a short while the above discussed failure mechanics. If a method or process were found that could ameliorate these problems, gas lasers could be made more rugged, smaller, at a lower price and have a longer life. The economics of such an event would open the use of lasers to many now prohibitively expensive situations as well as to lower the cost of current usage significantly. These two problems with lasers have persisted since their early development, i.e., the leakage of helium through the glass walls of the standard design gas lasers in general use which use helium as one of the gas components; and the control of the thermal environment in nearly all lasers whether or not they use gas as a component.
The struggle with the above two problems has led to engineering solutions which are basically techniques of living with the prime difficulties. These solutions include:
1. larger volume gas reservoirs to provide more gas and achieve longer life, PA1 2. using glass with the lowest available, but finite, helium leak rate, PA1 3. using ceramics and vitreous ceramics for laser bodies, PA1 4. for crude thermal control, wrapping aluminum foil loosely around the laser body. This foil distributes the heat, by conduction, along the length of the laser body thus reducing thermal gradients. The foregoing solutions are inefficient and make no direct approach to the physics of the base problems. PA1 "The final design of the space-qualified He-Ne laser utilized both the RZ-2 coating and the two-chamber technique. Analytically, this requires a redefinition of equations (6) and (7) in terms of the permeability of the RZ-2 glass and the fused silica. With the addition of the exterior RZ-2 coating, analysis indicates that operating periods in excess of ten years are feasible, thereby effectively eliminating helium depletion as a cause of failure in fused silica helium-neon lasers." PA1 "Hellium-neon lasers are supplied with mirrors mounted by a hard seal rather than epoxy, according to Hughes Aircraft Company's Industrial Products Division. Met-L-Glas seals employ solder glass, controlled surface interfaces and selected materials to create a structural vacuum seal between the laser mirror and the discharge chamber. Because the hard seals are impervious to water, they increase shelf life of the laser tubes to the time required for the helium to diffuse through the glass. Hard seals are standard on Hughes' He-Ne lasers priced at about $200 to $1100, delivery is from stock." Thus, up to the present time, the problem of life in helium gas lasers remains unsolved. The industry is, at the moment, using pyrex glass for laser bodies in a large reservoir design configuration. Although its thermal characteristics are not optimum, it allows less diffusion than fused silica and is cheaper. Using these current solutions, a low priced ($100-$125) laboratory laser will last about one year.
U.S. Pat. Nos. 3,437,950, 3,445,785, 4,007,431 and 4,017,808 all restate with various degrees of emphasis the problem of laser life and thermal control. U.S. Pat. No. 3,437,950 uses a shrink-fitted metal structure over a ceramic laser tube to carry away heat. The phenomenon which allows the shrink-fitted metal sleeve to work is the fact that the shrink-fitted sleeve is in tension. As the laser is heated and the material below expands, this tension is somewhat relaxed thereby keeping the stress on the under material within reasonable limits. The materials then have no great tendency to debond under normal thermal stresses. However, this is true for all metal-over systems where there symmetry in the structure and an under material of significantly lower coefficient of thermal expansion.
U.S. Pat. Nos. 3,445,785 and 3,979,696 both discuss coating of various parts of lasers to achieve greater efficiency. Both deal with optically pumped lasers coated with transparent material which absorbs off-axis radiation which can depump the laser. They point out that the well-known sputtering technique of coating is quite useful for depositing desirable layers onto solid state lasers or into the pumping cavity of an optically pumped device.
U.S. Pat. Nos. 4,007,431 and 4,017,808 both address laser life as a significant and largely unsolved problem. Both address the laser cathode in their search for a solution; one by design, one by coating through anodization.
With regard to helium leakage in lasers, the problem has existed since the development of gas lasers. In 1954, W. A. Rogers, et al, writing in the Journal of Applied Physics 25 868-75, studied the problem of helium diffusion through thick walled vessels. They derived an equation under the assumptions of a partial pressure of helium on one side of the vessel wall and none on the other. It should be noted that helium diffusion depends on the character of the material containing it and the partial pressure of helium on each side. The partial pressure of helium in the atmosphere is near zero. Regardless of the pressure of the other gases in the atmosphere, the contained helium thinks that the outside of the vessel is in a vacuum.
In 1961, E. L. Williams published in an Owens-Illinois Technical Center Report titled, "A Literature Review of Diffusion Studies in Glass", data on rates of diffusion of helium through various glasses. In 1970, W. N. Peters and E. K. Stein published in the British Journal of Physics E: Scientific Instruments 1970, Volume 3, a paper on work they had done on a NASA contract. That contract, NAS 12-502. was between the NASA Electronics Research Center and the Perkin-Elmer Corporation, Norwalk, Connecticut. The journal paper, "Helium Permeation Compensation Techniques for Long Life Gas Lasers", describes two approaches to the problem of helium containment and prolonging the life of gas lasers for space use. One technique used a double chamber wherein a large volume of helium was contained at high pressure. This helium diffused through glass membranes into the lasing chamber at a rate governed by the thickness of the membranes and the overpressure in the containment chamber. The membranes' thickness kept the helium pressure in the lasing chamber from getting too high. Helium also leaked through the outer walls at a known rate but since these outer walls were thicker and the diffusion slower, the operational life of the laser was extended. No effort was made towards totally containing the helium or using the glass envelope as a helium reservoir.
The second technique in this paper involved coating a basic laser with a glass of lower helium permeability, again using the data from Williams' 1961 report. That glass, RZ-2, is a copper doped glass from Owens-Illinois Inc. It has a thermal expansion coefficient compatible with fused silica and a significantly lower melting point. These latter characteristics are the goal and effect of the copper doping and permitted the material to be put on the outside of the fused silica laser body as a powder and melted in-situ. The RZ-2, upon solidification, would form a less permeable barrier to helium leakage. This paper noted that, according to Williams, the permeability of RZ-2 was nearly four orders of magnitude less than fused silica. They found that even a "relatively thin coating of RZ-2 on all of the exterior surfaces of the fused silica laser body would dramatically reduce permeation."
The conclusion of that paper follows:
The solution arrived at in the Peters paper has two major flaws which have mitigated against its adoption since it was generated.
1. The solution is uneconomic, and
2. The solution is partial in application. As stated in the Peters paper, helium permeation was "reduced". However, in order to achieve this reduction, which was never quantitized, it was necessary to use double walled chambers with walls of controlled known thickness. Producing single glass chambers with controlled wall thickness is not difficult. But, double walls, at least in the design configuration of the Peters paper, are difficult and expensive and do not lend themselves well to known production techniques. No application of the Peters solution has been found, and the glass envelope was not used as a helium reservoir.
In the March, 1978, issue of Laser Focus magazine, an ad appears on page 70 as follows:
In addition to the life limiting leakage of helium, there is a thermal problem existing with all lasers. In gas lasers, the plasma generates heat which must be distributed and dissipated. The materials normally used in laser construction generally have a low coefficient of thermal conductivity and the laser body designs are long and slender. Both of these characteristics contribute to large thermal gradients and hot spots which result in optical misalignments and changes in the optical characteristics of the laser output beam. One solution to the problem currently used is to wrap the lasers in aluminum foil. Although this method is adequate for some operations, it is inefficient because intimate contact does not exist between the foil and the laser body.